We have seen the latest recently when the free web-page provider Geocities decided to stop permanently.A number of, for us winemakers, important web-pages would potentially get lost. For example both the books from Lum Eisenman and Michel Pesgens (who also has a great wine calculator on his pages) where located at Geocities and were potentially lost.Both have a free downloadable book and other valuable information on their web pages.

Fortunately I had contact with both authors and they have relocated their books and other info to new sites.

These sites can be found on the right side of this web-log under the heading: FREE DOWNLOADABLE WINEMAKING BOOKS

This book, which was also free downloadable, was located at:http://home.thezone.net/%7Ephumber/netscape.html

However the complete provider thezone.net seems to have vanished. This might be a victim of the economic crisis.

The only hope that I have is that amongst te thousands of readers of this web-log there will be at least one that can bring me in conatct with the author, or who has a new location (URL) for this book.

It would be a pity if this books was lost forever. Not just because the author undoubtly put a lot of effort in it, but also because free information is invaluable for the beginning winemaker.

Just to make sure, I am not looking for the book itself. I already have it on my own harddisk. I am looking for a legal link where I can point interested winemakers to if they want to download the book. I am not offering it myself as I am not willing to violate any copyright issues.

This is the fourth part in my series about measuring acidity in a must or juice. This part describes how to measure acidity in a dark red must or juice by using the reverse titration method.

Part 1 showed how a normal titration is used to measure acidity in a white wine/must. This method is indeed fine for white musts but not suitable for reds as it is difficult to see when the actual colorchange happens.

Part 2 showed how you can dillute a red must to show the colorchange during titration a lot better. This method makes it possible to measure acidity by titration accurately. It is however a method that involves some handling and calculations.

Part 3 examined the method which, is actually advocated in some winemaking magazines, de-colored a must using active coal. The tests clearly showed that filtering a must with active coal not only strips it from color but also de-acidifies it. This makes this method totally unreliable.

This part shows you how to measure accurately the acidity in a dark red must or wine by using the 'reverse method' in titration. This method makes measuring acidity in a dark red must a lot easier, as almost no calculations are needed. The theory is maybe a bit complicated to follow if you are not a chemist, however in real-life this method is very easy to use. Just follow the samples and you will know what I mean.

Needed for this method:

- An acid titration kit- A pipette- Dsitilled water

If obtaining distilled water is a problem you can use demineralised water or reversed osmosis water.

The theory behind it.

The theory behind this method presumes that when performing a titration we use a known amount of juice, must or wine. In this volume there will be a certain amount of acid. Titration is like counting the hydrogen ions. By adding distilled water, the volume will increase but the total amount of hydrogen ions will stay the same.The theory says that at titration hydrogen-ions will be neutralised. However by adding distilled water we increase the volume but we do not increase the amount of hydrogen ions.Saying thigs in another way: the juice will be dilluted but will contain the same amount of ingredients (except H20 of course). And the blue-indicator will act on the amount of hydrogen ions, which is not altered.

Mind you, this will off course only work with distilled water, demineralised water or reverse osmose water. Normal tap-water will have all kinds of minerals in it that will influence the measurements.

Real life tests

All this theory seems quite complicated however in real life it is very simple.

First we fill the measuring tube of the acid-titration kit to the 0 marker with the must we want to test. We pour this sample in an empty glass and add distilled water until the color has lightened to our liking. The amount of distilled water is of no importance. Just pour in as much as you think will be needed.Next we add blue indicator until there is a distinct color change. By measuring how much blue-indicator we have added we will know the acidity.

Here we go.

I started with a red wine. For testing purposes I performed a normal titration first. This wine was not a real dark wine so any color change should be noticed.

As with any normal titration (see my first article on this) I filled the testing tube to the 0-marker and added blue indicator until the color had changed.

As you can see the color changed at the 7 mark.

So I had a reference and started the 'reverse-method'

First I filled the test tube again with the wine till I reached the 0-marker.

Next this sample was poured into an empty glass.

The testing tube was then cleaned and dried (you do not want any wine left in it) and filled to the 20-mark with blue indicator.

Next I poured distilled water in the glass with the sample. I used as much distilled water until the colour of the wine had changed to light-red. It does not matter how much distilled water you add. The theory tells us that the total amount of acid in the fluid will not change as distilled water does not contain any acid, minerals or whatever.

Using a pipette I removed blue indicator form the testing tube until the level dropped from 20 to 19.

Next the blue-indicator in the pipette was added to the glass with dilluted wine. I mixed thoroughly with a spoon. Nothing happened. The colour did not change,.

Again I took some blue indicator out of the testing tube till the level had dropped another mark. And this was also added to the glass.

I continued these steps.

When the level blue-indicator in the testing tube dropped to the 14-mark the colour in the glass darkened.

I repeated the steps one more time and at the 13-mark the wine coloured dark. So the dillution made the colour change perfectly visible.

So what was the acidity of this wine ??

Well I started by filling the test-tube with blue indicator to the 20-mark. When the color change was obvious the level was at the 13-mark. So I had used 20 -13 = 7 units of blue indicator. Exactly the same amount as with a normal titration.

It should be clear now why we call this the reverse-method. Normally you add blue indicator to the test tube, with this method you remove blue-indicator from the test tube and calculate the difference.

Another test: apple-blackberry wine

The previous test indicated that this method works accurately. However just one test is off course no prove for a serious investigation. So I performed another test, using an apple-blackberry wine.

First I did a normal titration for reference purposes.

Like always I started with filling the test-tube till the 0-marker.

Next I added, step by step, blue indicator until the colour change was clearly visible. To my surprise the wine had an acidity of 9. Much higher as I would have expected from the taste of it.

Then I practiced the reverse-method.

Again the test-tube was filled with wine till the 0-marker.

Next the test-tube was emptied in a glass and I added distilled water until the colour was light enough in my opinion.

Again the test tube was filled with blue-indicator to the 20-mark. I repeated the steps from the previous test by step-by-step adding blue-indicator to the dilluted wine in the glass. I continued the steps until the dilluted wine started to change color.

When the level indicator in the test-tube was at the 12-mark the wine was a lot darker.

At the next step the indicator was at the 11-mark and the wine definately coloured dark. Again the colour change was now really obvious.

So now I was able to determine the acidity of this wine. The level in the testing tube started at 20. When the colour change happened the level was at 11.So acidity was at 20 -11 = 9

Again the reverse-method gave exactly the same outcome as a normal titration.

The last test: A predefined measurement of acid.

The ultimate test for accuracy is of course performing the titration with a fixed amount of acid.

Now this can easily be accomplished. As a winemaker I have all kinds of acid in stock: citric-acid, malic acid and tartaric acid.To test accurately I used tartaric acid.

Why tartaric acid ??Well to raise acidity in a wine by 1 you would add 1 gram tartaric acid to 1 liter must.The titration kits we use are based on this. They measure acidity in grammes tartaric acid. Titration kits can not determine what kind of acid is in the fluid. So they re-calculate acidity in the equivalent of tartaric-acid.Malic acid and citric acid will raise acidity in a liter more as 1 point as 1 gram is added. How this can cause great consequences can be read here.

So I used tartaric acid.

First using a precision scale I measured exactly 7 gram tartaric acid.

The acid was put into a measuring beaker and I filled it with water to the 1 liter mark. Now I had a fluid with an exact acidity of 7.

Again I started a normal titration as a reference. The testing tube was filled to the 0-mark with the acid water. Next titration was performed until the colour changed which off course happened at the 7-mark.

Now the reverse-method.

The test tube was filled to the 0-marker with the acid water.

Next the test-tube was emptied in a glass and I added some distilled water.

Again the amount of distilled water is not important. I refer to the theory that states that by adding distilled water the volume will change but not the total amount of acid-ions.

After cleaning the test-tube it was filled to the 20-mark with blue indicator.

Step by step blue indicator was removed from the test tube and poured into the dilluted acid water. Each time the level in the test tube dropped 1 mark and each time the fluid in the glass was stirred well to mix the blue indicator thoroughly.

As you can see the colour changed at once when the level in the test tube reached the 13-mark !!!

Again calculating acidity is easy.I started with blue-indicator at the 20 mark, and ended at the 13 mark. So acidity will be at 20 - 13 = 7.Exactly the amount of acid I measured with the scale.!!!!

Summarising

When measuring acidity in a dark must or wine, the colour change is often difficult to see which makes a normal titration unreliable.

By using the reverse-method you will the fill the test tube with must or wine till the 0-marker (as usually). Next you will pour this quantity into an empty glass and add distilled water until the colour of the must/wine becomes light enough.Then the testing tube is filled to the 20 mark and you will step-by-step take blue indicator out and add that to the dilluted wine/must. Continue untill the colour change is clearly visible.

Now you can determine the acidity of tyhe must or wine by subtracting the level in the test-tube at the colour change from the starting level (20).

Concluding:

Using the reverse-titration-method we have foud not only an accurate but also easy mehod to test acidity in a dark must or wine.

The only extra's we will need beside a normal titration kit is a pipette and some distilled water.

Distilled water can often be found at the household department of your local warehouse (needed for steam-ironing) or at the local gas station as battery-water.

When distilled water is not available you can use demineralised water or reverse osmosis water.